1. Field of the Invention
This invention relates in general to sensors for reading magnetic flux transitions from magnetic media such as disks and tapes, and more particularly to a method and apparatus for re-initialization of a GMR head after losing initialization.
2. Description of Related Art
An MR sensor detects magnetic field signals through the resistance changes of a magnetoresistive element, fabricated from a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the element. The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the element resistance varies as the square of the cosine of the angle between the magnetization in the element and the direction of sense or bias current flow through the element.
MR sensors have application in magnetic recording systems because recorded data can be read from a magnetic medium when the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR read head. This in turn causes a change in electrical resistance in the MR read head and a corresponding change in the sensed current or voltage.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR), has been observed in a variety of magnetic multilayered structures. The essential feature of a GMR sensor is that is contains at least two ferromagnetic metal layers that are separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr or Co/Cu multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers, as well as in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferromagnetic layers is fixed or pinned. The physical origin is the same in all types of GMR structures: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus a change in the electrical resistance of the structure. The resistance of the structure changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of the GMR effect is a sandwich structure comprising two only weakly coupled ferromagnetic layers separated by a nonmagnetic metallic spacer layer in which the magnetization of one of the ferromagnetic layers is pinned. The pinning may be achieved by depositing the ferromagnetic layer to be pinned onto an antiferromagnetic layer, such as an iron-manganese (Fe--Mn) layer, to create an interfacial exchange coupling between the two layers. The spin structure of the antiferromagnetic layer can be aligned along a desired direction (in the plane of the layer) by heating beyond the blocking temperature of the antiferromagnetic layer and cooling in the presence of a magnetic field with a predetermined direction.
The blocking temperature is the temperature at which exchange anisotropy vanishes because the local anisotropy of the antiferromagnetic layer, which decreases with temperature, has become too small to anchor the antiferromagnetic spins to the crystallographic lattice. The unpinned or free ferromagnetic layer may also have the magnetization of its extensions (those portions of the free layer on either side of the central active sensing region) fixed, but in a direction perpendicular to the magnetization of the pinned layer so that only the magnetization of the central region of the free-layer can rotate in the presence of an external field. The magnetization in the free-layer extensions may be fixed by longitudinal hard biasing or exchange coupling to an antiferromagnetic layer. However, if exchange coupling is used, the antiferromagnetic material is different from the antiferromagnetic material used to pin the pinned layer, and is typically nickel-manganese (Ni--Mn). This resulting structure is called a spin valve (SV) MR sensor. However, for clarity, the term GMR sensor will be used herein to denote both spin valve as well as GMR sensors.
In a GMR sensor only the free ferromagnetic layer is able to rotate in the presence of an external magnetic field. U.S. Pat. No. 5,159,513, assigned to IBM, discloses a GMR sensor in which at least one of the ferromagnetic layers is of cobalt or a cobalt alloy, and in which the magnetizations of the two ferromagnetic layers are maintained substantially perpendicular to each when an externally applied magnetic field is not present by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer.
U.S. Pat. No. 5,206,590, also assigned to IBM, discloses a basic GMR sensor wherein the free layer is a continuous film having a central active region and end regions. The end regions of the free layer are exchange biased by exchange coupling to one type of antiferromagnetic material, and the pinned layer is pinned by exchange coupling to a different type of antiferromagnetic material.
GMR sensors are a replacement for conventional MR sensors based on the AMR effect. They have special potential for use as external magnetic field sensors, such as in anti-lock braking systems, and as read heads in magnetic recording systems, such as in rigid disk drives. However, the GMR sensor, which is typically fabricated by depositing an antiferromagnetic layer of Fe--Mn onto the ferromagnetic pinned layer of cobalt (Co) or permalloy (Ni--Fe), suffers from the problem that the range of blocking temperature for this interface is relatively low, i.e., it extends only from approximately 130.degree. C. to approximately 160.degree. C. These temperatures can be reached by certain thermal effects during operation of the disk drive, such as an increase in the ambient temperature inside the drive, heating of the GMR sensor due to the bias current, and rapid heating of the GMR sensor due to the head carrier contacting asperities on the disk. In addition, during assembly of the disk drive the GMR sensor can be heated by an electrical current resulting from an electrostatic discharge often referred to as electrical overstress.
If any of these thermal effects cause the GMR sensor to exceed the antiferromagnet's blocking temperature the magnetization of the pinned layer will no longer be pinned in the desired direction. This will lead to a change in the GMR sensor's response to an externally applied magnetic field, and thus to errors, in data read back from the disk.
Nevertheless, the height of MR and GMR sensors used in magnetic recording is determined by mechanical lapping. This mechanical machining process results in wide tolerances in the sensor height. Typically a sensor of 1.5 .mu.m average height may vary from sensor to sensor from 0.7 .mu.m to approximately 2.3 .mu.m. The sensor resistance therefore also shows a wide tolerance from sensor to sensor. If such a sensor were reset by means of a current pulse, the dissipation I.sup.2 R would be the highest in the highest resistance sensor, i.e., the sensor with the lowest sensor height. The temperature rise in the latter sensor would also be the highest.
If, instead the sensors are reset by a voltage pulse, the resulting current density is the same in all sensors independent of their height. The temperature rise of the sensor due to the pulse can be calculated to be proportional to the square of the current density. Therefore, all sensors, reset with a voltage pulse, would experience the same temperature increase regardless of their sensor height.
It can be seen that there is a need for re-initializing a GMR head after losing initialization by using a voltage waveform rather than a current pulse.
It can also be seen that there is a need for a method for re-initializing a stack of GMR heads so that the GMR heads all have the same orientation thereby precluding the inversion of half of the GMR heads facing oppositely from the rest.
It can also be seen that there is a need for an apparatus that can provide re-initialization of GMR heads at different points in the manufacturing process.